The present invention relates to a compound rotary encoder for detecting the rotation angle and the number of rotations of a rotating disk connected to a rotor shaft.
Prior-art rotary encoders for detecting the rotation angle of the rotor shaft include both incremental and absolute rotary encoders. An incremental rotary encoder has evenly spaced binary-code patterns on the rotating disk connected to the rotor shaft. While the rotating disk rotates, the binary code patterns are detected as pulse signals, the pulse signals are counted, and thus the rotation angle of the rotor shaft is detected.
The patterns can be easily formed on the rotating disk of the incremental rotary encoder. The resolution of the pulse signals can be enhanced by increasing the number of binary-code patterns However, when the rotor shaft rotates slowly, the interval of pulsation lengthens, and the precision of control is impaired. Moreover, detected values often deviate from correct values due to electrical noise. On the other hand, the absolute encoder can instantly detect the absolute position corresponding to the rotation angle from its sensor without using a counter.
A representative detecting board for the absolute encoder is a rotating disk. On the face of the rotating disk, concentric circular patterns are formed whose number corresponds to the number of signal bits. Binary patterns are detected from these circular patterns
In the absolute encoder, when n denotes the number of signal bits, the resolution is 2.sup.n. For example, when ten sensors are used, the rotary encoder has a resolution of 2.sup.10 =1024. However, to enhance the resolution, more sensors and finer circular patterns are required, thus increasing the cost of the encoder. Moreover, when the rotating disk is reduced in size, the circular patterns decrease in diametrical width, and the information in the circular patterns becomes harder to detect.
A compound prior-art rotary encoder is disclosed in the Japan Published Unexamined Patent Application No. S60-100015. The rotary encoder has both incremental code patterns and circular absolute code patterns on its rotating disk. The rotary encoder detects the rotation speed of the rotor shaft using the incremental code patterns and detects the rotation angle of the rotor shaft using the absolute code patterns. The signals detected from the incremental code patterns are processed independently of the signals detected from the absolute code patterns. Consequently, the number of absolute code patterns must be increased to detect the rotation angle of the rotor shaft with higher resolution. The accuracy of the rotation angle detected from signals may vary due to errors in the detection of signals or due to the missing of pulses.
This prior-art rotary encoder detects the rotation angle of the rotor shaft as the number of rotations by decelerating the rotor shaft with gears and other mechanism.
Since the encoder has the system for decelerating the rotor shaft for the detection of the number of rotations, it cannot be reduced in size, and many components are required.
In this way, the prior-art rotary encoder detects the number of rotations of the rotor shaft independent of the rotation angle. The detected rotation angle does not coincide with the detected number of rotations.
For example, although the detected rotation angle is only 359 degrees, the rotation count might increase. Moreover, although an additional rotation angle of one degree is detected, the number of rotation might not increase. This problem increases in severity, when the resolution of the rotary encoder increases.
The prior-art rotary encoder converts the number of rotations as well as the rotation angle from parallel signals into serial signals, and transmits the signals to a motor controller, where the serial signals are converted back into parallel signals. Parallel signals are used for control. By using serial signals to transmit, the number of transmission wires can be reduced.
However, when the resolution of the rotation angle signals increases, the time for detecting a least significant bit of the rotation angle signals shortens due to the time required for transmitting and converting serial signals. The transmission and conversion of signals delay the detection of the rotation angle signals. Since signals must be transmitted rapidly, some bits are lost during the transmission of signals. The reliability in the transmission is thus impaired. On the other hand, when parallel signals are transmitted to the motor controller without being converted to serial signals, the number of transmission wires would increase.